Variation of viscoelastic properties of extracellular polymeric substances and their relation to anaerobic granule’s mechanical strength in full-scale treatment plants

Extracellular polymeric substances (EPS) are considered to play a pivotal role in shaping granules' physical properties. In this contribution, we characterized the viscoelastic properties of EPS from granules of 9 full-scale industrial anaerobic reactors; and quantitatively investigated whether these properties correlate with granules' resistance to compression (E_granule) and shear strength (S_granule). Most granules with a higher shear strength, also exhibited a stronger resistance to compression (r = 0.96, p = 0.002), except those granules that contained relatively more proteins in their EPS. Interestingly, these granules were also the most resistant to shear stress (S_granule ≥ 110 ± 40 h). Furthermore, the EPS hydrogels of these granules had slower softening rates (κ < 0.9) compared to the others (κ ranged between 0.95 and 1.20), indicating stronger gels were formed. These findings suggest that the EPS hydrogel softening rate could be a key parameter to explain granule's shear strength.